Safe operation of an elevator system in the event of a malfunction, such as an electricity outage, is ensured with the machinery brake of a hoisting machine. In addition, so-called dynamic braking is often used in elevator hoisting machines, in which case the windings of the hoisting machine are short-circuited with dynamic braking switches, e.g. during a standstill of an elevator. Racing of the hoisting machine can be prevented with a short-circuit of the windings because when the hoisting machine moves the source voltage induced in the short-circuited windings produces a current that endeavors to brake the movement of the hoisting machine. Most of the electrical energy produced in dynamic braking in this case is converted to heat in the winding resistances of the hoisting machine.
Instead of short-circuiting the windings, the electrical energy produced in dynamic braking can also be supplied to a load outside the hoisting machine, such as to a power resistor. In this way heating of the hoisting machine can be reduced during dynamic braking. The power resistor needed is, however, generally rather large in size; in addition, a free space must be reserved around it owing to the strong heat rise occurring in the resistor.
The contacts of a contactor, for instance, can be used as switches of dynamic braking. In certain exceptional situations, such as in connection with brake tests, the dynamic braking function must be temporarily removed from use. Sometimes the function is removed from use by detaching the contacts of the contactors of dynamic braking from the supply cables of the hoisting machine. In this case there is a danger that it is forgotten to re-connect the contacts of the contactors of dynamic braking after the tests have been performed. Without dynamic braking, an elevator car may race when the brake is left free, so forgetting the re-connection of the contacts of the contactors of dynamic braking may cause a dangerous situation to a serviceman working in the elevator hoistway.
Instead of the contacts of the contactors also the solid-state switches of an inverter controlling the hoisting machine, can be used as switches of dynamic braking. A substantially high current may flow in the solid-state switches during dynamic braking. The current causes considerable warming in the power semiconductors, which again could shorten the service life of the inverter. For this reason, the current stress caused to the solid-state switches by dynamic braking must also be taken into consideration when dimensioning the inverter.